Manufacturing formulations involving aqueous solid dispersions often require to be processed at a high concentration of solids for one or more of the following reasons: to improve the costs/efficiency of transportation, to increase laydown or to reduce the drying load, as the removal of water is wasteful and expensive. The formulations also need to be stable over a wide range of shear, which extends from low to high shear rates. After mixing, aqueous dispersions of particulates, especially those containing high concentrations of solid, tend to form a ‘yield stress material’ as the shear stress is relaxed, the viscosities climbing dramatically with the decreasing shear such that, for example, gels may be deposited in zones of reduced shear. The high viscosity of the dispersions at low shear affects their dispersability and flowability, making the systems difficult to stir, pump, transport, coat or pour. Moreover their tendency to form a gel or a yield stress material in regions of low flow is often more of a problem, as gelation is often irreversible and consequently totally unacceptable for manufacturing processes that rely on systems to flow.
It is known in the art that an inorganic, organic or polymeric form of acid may be added to an aqueous dispersion of alumina. Thus U.S. Pat. No. 3,935,023 describes the use of hydrochloric acid to form stable dispersions of alumina, whilst EP-A-0 736 491 teaches the use of a viscosity-reducing agent for an alumina sol based on a compound having a carboxyl group or a sulfonic acid group, or a compound selected from the group consisting of formamide or acetamide, relating to small non-polymeric molecules. United States Patent Application No. 2002/0027304 discloses the use of acetic acid, citric acid or a polyacrylate as a viscosity modifier for alumina systems.
DE 19703582 describes the use of carboxylic acid and salts for reducing the viscosity of aqueous pigment dispersions of zinc oxide and titanium dioxide pastes. WO 9715382 discloses that metal oxides can be dispersed in an aqueous medium using a hydrolyzed polymer containing maleic anhydride for use in coatings and cosmetics and BE 903772 and U.S. Pat. No. 4,687,789 describe an acrylic acid/itaconic acid/methacrylic acid copolymer for stabilization of a zinc oxide or titanium dioxide dispersion for providing rheological stability.
EP-A-0 697 998 teaches the combination of zinc oxide with a polyacrylic acid dispersant to provide a colloidal-sized zinc oxide material. WO 2003/037984 describes a water-soluble polyelectrolyte/polyacid copolymer for use as an inorganic pigment dispersant for dispersing zinc oxide and zinc oxide/titanium dioxide slurries. EP-A-1 207 143 discloses methods for dispersing and binding ceramic materials in aqueous media using water-soluble polymers having pendant derivatized amide, ester or ether functionalities.
U.S. Pat. No. 4,548,733 discloses a method of reducing the viscosities of aqueous slurries of particulate materials, such as titanium dioxide, silicon dioxide, iron oxides, cobalt-modified iron oxides, aluminum oxide, antimony oxide, chromium dioxide, zinc oxide, zinc sulfide, basic carbonate white lead, basic sulfate white lead, barium sulfate, calcium carbonate, magnesium silicates, aluminum silicates, mica and clays, by using anionic siliconates of silylorganocarboxylates, sulfonates and phosphonates.
WO 2004/000916 describes a process to prepare a stable dispersion of nanoparticles in aqueous media using a copolymer/cyclic phosphate dispersant, and the use of an acrylamide polymer for flocculating the boehmite form of alumina is disclosed in U.S. Pat. No. 3,117,944.
Copending UK patent application No. 0428260.4 discloses the particular use of a dispersant to reduce the viscosity and/or tendency to gel of, specifically, the boehmite form of alumina which has an unusual rheological character which is dependent on its shear stress history.
There are a number of publications which further teach the use of acids as dispersion stabilizers, such as, for example, ‘Ceramic Development: Programme Research at the Swedish Ceramic Institute 1993-1996’ and Journal of Ceramic Processing Research, 3 (2002) 10-14 ‘Suspension Systems for Coagulation Processing’ by C. Pagnoux, which discloses the use of a low molecular weight sulfonic acid (TIRON) and a polyacrylic acid (DISPEX N40).
P. C. Hidber et al., in Journal of the European Ceramic Society, 17 (1997) 239-249 ‘Influence of the Dispersant Structure on Properties of Electrostatically Stabilized Aqueous Alumina Suspensions’ describes a dispersant based on a hydroxy- and carboxy acid-substituted benzene ring. A. U. Khan et al., J. Mater. Chem., 12 (2002) 1743-1747 ‘The Strength of Colloidal Interactions in the Presence of Ceramic Dispersants and Binders’ discusses the use of the ammonium salt of aurintricarboxylic acid (ALUMINON) as a dispersant of alumina dispersions.
The use of citric acid, gallic acid and 2-phosphonobutane-1,2,4,-tricarboxylic acid, as dispersants for alumina for producing zero cement high-alumina castables for refractory purposes is described in A. R. Studart et al., Journal of the European Ceramic Society, 23 (2003) 997-1004 ‘Selection of Dispersants for High-Alumina Zero-Cement Refractory Castables’. P. C. Hidber et al., in J. Am. Ceram. Soc., 79[7] (1996) 1857-67 ‘Citric Acid—A Dispersant for Aqueous Alumina Suspensions’ discusses the science of the high affinity of citric acid for an α-alumina surface.